The present invention relates to an open-magnetic-circuit-type engine igniting coil device.
Japanese Utility Model Publication No. 4-23296 discloses an open-magnetic-circuit-type engine igniting coil device which has a coil case, in which an ignition coil assembly consisting of a primary coil bobbin with a rod-shaped core inserted in its hollow center and a secondary coil bobbin coaxially surrounding the primary coil bobbin is mounted and integrally potted with melted insulating resin, and has an ignition-plug connector integrally formed on the coil case to allow a tip of an ignition plug to contact with a high-voltage terminal inwardly projecting in the connector portion.
Usually, melted insulating resin is injected into a slender cylindrical coil case in pre-evacuated state. In this case, it is needed to fill the coil case with an excessive amount of the liquid resin because poured resin is further drawn into the coil case when the latter is exposed to an atmosphere pressure.
In the conventional engine igniting coil device, an output terminal 71 of a secondary coil shown in FIG. 9 is connected by fusion to a high-voltage terminal 12' having a U-shaped cross-section, which is attached to a secondary coil bobbin 8'.
In another conventional arrangement shown in FIG. 10, an output terminal 71 of a secondary coil is wound on and soldered to a projecting high-voltage terminal 12' attached to a secondary coil bobbin 8'.
Japanese Laid-Open Patent No. 4-143461 discloses another engine igniting coil device comprising a cylindrical coil case having a high-voltage terminal connector in its open-bottom end and incorporating a coil assembly consisting of primary and secondary coil-wound bobbins with a core inserted in a hollow center of the coil bobbin and integrally potted therein with melted insulating resin, which is mounted in a bore in a cylinder head of an engine and is connected at its connector with an ignition plug of the engine.
The above-mentioned prior art devices, however, involve the following problems to be solved:
The first problem is that the conventional open-magnetic-circuit type engine igniting coil device having the rod-like core inserted in a hollow center of the coil assembly consisting of primary coil-wound and secondary coil-wound bobbins may allow a magnetic flux produced therein to spread outwardly and lose part of the magnetic flux when passing through the cylinder block of the engine, resulting in decreasing the output factor of the secondary coil. Consequently, the device must be larger to obtain a desired secondary output voltage.
An attempt to prevent spreading of the magnetic flux produced in the device by covering the coil case with magnetic plates was accompanied by a leakage-current discharge from the high-voltage portion to the magnetic plates.
The second problem is that an amount of melted insulating resin injected into an engine igniting coil device may be variable and an excess of melted resin may be spilled out and contaminate the outer surface of the coil case while the latter is transported to a curing furnace. To avoid this, it is necessary to increase the volume of the coil case.
In the coil case, residual air may form bubbles of melted resin, which may spray out and contaminate the outer surface of the coil case.
The cylindrical coil case having a narrow opening and long body can not be entirely filled with melted resin if air is left and shut in the coil case. Therefore, melted resin is poured gradually little by little into the coil case, which takes much time.
The third problem is that a conventional engine igniting coil device which is mounted in a bore in a cylinder head of an engine and attached directly to an ignition plug of the engine may be subjected to vibration of the engine and, therefore, requires the provision of means for decreasing the vibration transmitted therefrom.
The engine igniting coil device mounted in a bore in a cylinder head of an engine may also be subjected to a large thermal stress in an axial direction of its coil case and requires the provision of means for absorbing an axial thermal elongation and contraction of metal.
The fourth problem is that an engine igniting coil device has a large terminal connection. Typically, an output terminal of a secondary coil is connected by fusion to a U-shape type high-voltage terminal or by soldering to a projecting type high-voltage terminal attached to a secondary coil bobbin. Both terminal connecting means must be located outside of the secondary coil bobbin and separated from the coil case to provide a necessary insulation distance. This may increase the size of the engine igniting coil device.